a. Field of Invention
This invention pertains to under water seismic exploration and more particularly to apparatus for generation of underwater acoustic impulses. The inventive feature resides in a high voltage solid state electrical switch for discharging a capacitor through an underwater electrode system.
B. Description of the Prior Art
The use of an electric discharge between a pair of electrodes in a liquid medium for the generation of waves of pressure and motion in the liquid is well known and is described in U.S. Pat. No. 2,167,536 (Suits), U.S. Pat. No. 3,230,506 (Hellund), U.S. Pat. No. 3,416,128 (Allen), U.S. Pat. No. 3,283,294 (Schrom).
The use of such a wave source in connection with underwater seismic exploration is described in U.S. Pat. Nos. 2,994,397, 3,251,027 and 3,286,225 to Huckabay et al, and in U.S. Pat. No. 3,286,226 to Kearsley, and U.S. Pat. No. 3,479,638 to Rusnak. Such apparatus so used is known as a sparker.
One or more suitable tip electrodes may be used in conjunction with an untipped or common electrode according to U.S. Pat. No. 3,245,032 (Knott et al), U.S. Pat. No. 3,537,542 (Dubois). See also Technical Bulletin 652 published by Edgerton, Germeschausen & Grier, Inc.
The use of a plurality of pairs of electrode tips is known to the prior art and is currently illustrated in a brochure entitled "The Teledyne Exploration SUBot T.M. Subsea Profiler for Geologists and Engineers" published by Teledyne Exploration Company. See also "Operation and Maintenance Manual Energy Storage System, Model 27290", dated May 7, 1968, by Teledyne Exploration, "The Generation of Underwater Acoustic Impulses with Electrical Discharge Technique" dated June 1968, by W. H. Luchrmann; and the 1967 and 1968 brochures of Geotech Teledyne referring to the "SUBot" system and the Model 24218 Energy Storage System; and the Instruction Manual for the Model 24218 System; and more recently, Burrage U.S. Pat. No. 3,813,823.
Apparatus upon which the present invention is an improvement is portrayed in a publication by Teledyne Exploration Company entitled "Portable Seismic Source Model 250", although only exterior views of the equipment are given.
As indicated above, sparkers have been constructed in a variety of electrode configurations, electrical specifications and later developed into programmed sparking and exploding wire devices, etc. All of these systems have a common basic concept. First, there is an energy storage capacitor bank; second, a means to switch the stored electrical energy from the capacitor bank into a pair or pairs of electrodes at the command of a voltage trigger. Most of the switching devices at the required power level in a present day sparker system are either spark-gaps (open air plasmatic switch) or gases and vapors in an enclosed chamber which can be ionized at the command of a trigger. Because of the relative advantages of its performance, the spark-gap remains the most popular high energy switch used in the sparker systems of today.
Because of the fact that the triggered spark-gap is an open air switch (as opposed to a hermetically sealed switching junction), it has inherent drawbacks in its performance. The potential difference required across the gap in order to ionize it under the command of a specified trigger level depends upon such factors as the humidity and the temperature of air in the main gap, length of the main gap, the shape of the gap electrodes, etc. The ionization of the gap produces spot heating on the tips of the main gap electrodes resulting in its gradual degradation. Significant amount of electro-magnetic interference may be caused to the electronic equipment in the vicinity by the plasma generated in the main gap. Furthermore, the ionization of the main gap results in the loss of energy in the form of heat, light and acoustic noise. With so many factors affecting the ionization of the triggered spark-gap, it is difficult to maintain reliable trigger control of the ionization of the gap when the operating conditions change. Therefore, frequent adjustments in the length of the triggered spark-gap may be required during the start up of a sparker system when damp air may be residing in the gap and later as the changes in temperatures, humidity, power line voltages, and electrode shape due to erosion become significant. Spurious `misfires` and `auto-fires` can occur in the spark-gap if the gap length is not adjusted promptly.
It is an object of the invention to provide a high energy electrical switch that will neither have the problem of electrode erosion nor will generate any plasma or audible noise at the switching junction. The control of the switch will not be influenced by the changes in the humidity and temperature of the operating environment within the specifications. It is further the object of the invention to provide a high energy switch that requires no electrical or mechanical adjustments to itself or any service to itself during its operating life-span. It is a further object of this invention to provide a switch that will offer better power control than a spark-gap within the power limitations, and that will make a good substitute for spark-gaps in many applications of sparker systems used for high resolution seismic profiling of large water covered areas.